This invention relates to a single side-band frequency division multiplexed (SSB-FDM) communication system for carrying out conversion between a plurality of baseband channel signals, each having a predetermined bandwidth, and an SSB-FDM signal.
In a sophisticated communication system of the type specified, the conversion is carried out by the use of an analog modulator, demodulator, and band-pass filters. Recent developments in integrated circuit techniques and digital signal processing have enabled digital conversion to be resorted to. The digital conversion renders the system compact and economical, facilitates manufacture and ease of maintenance of the systems, and raises the performance and the uniformity of operation characteristics. Besides the enumerated merits, digital conversion makes it readily possible to carry out connection between a time division multiplexed (TDM) digital communication network and an FDM analog communication network. The compactness and the cost of manufacture of a system for performing digital conversion, however, are dependent on the frequency or rate of multiplication to be carried out in the system per unit time.
An example of the systems of the digital conversion type is disclosed in an article contributed by M. G. Bellanger and J. L. Daguet to "IEEE Transactions on Communications," Vol. COM-22, No. 9 (September 1974), pp. 1199-1205, under the title of "TDM-FDM Transmultiplexer; Digital Polyphase and FET," particularly with reference to FIG. 8 of the article. In the disclosed system, input sample sequences are converted to sequences of complex samples. The complex samples are supplied to an inverse discrete Fourier transform (IDFT) processor having N pairs of input terminals for the complex samples and 2N output terminals for real signals. The real signals are supplied to 2N real filters, whose output signals are converted into an output SSB-FDM signal by a delay circuit. Inasmuch as the input sample sequences are converted to complex signals before the IDFT processing, the disclosed system has to carry out multiplication at a considerable rate, giving rise to an accumulation of errors, and is bulky, and has a slow speed of operation.